I also don't understand and I don't think it was a good idea. In my opinion it totally removes the glamour and protagonism of a country as the registered brand of the World Cup's edition. When we talk about previous World Cups it's nice to attach each of them to a different country, as the culture of the countries which host the event become a symbol of their respective editions. Now with Canada, USA and Mexico hosting a World Cup, all of them at once, it becomes a confusing event with no original and memorable traits at all. Maybe they did this to cut expenses for the hosts, so the costs are splitted between the three parts, but strange anyway.
Wait until you see the application for 2030 hehehe, there's a good chance to have Argentina, Chile, Uruguay and Paraguay hosting the World Cup, specially because it will be 100 years of World Cup, and the first one in 1930 was in Uruguay.
They even tried to include Colombia as 5th country, but Fifa rejected
And the other application to host 2030 World Cup
is are: Spain, Portugal and Ukraine.
They included Ukraine because of the war situation, this is a strong reason to Fifa consider the application even more.
I think it's a good idea to have 2 countries or even more hosting the World Cup because one country maybe don't have all the infrastructure to make the whole World Cup, and it helps football spreading in even more countries.
But my ideal of World Cup would be every 2 years instead of 4 and with 32 countries.